In applications where variable speed electric motors are required, DC motors are generally utilized because of the ability to accurately control such motors over a broad range of speeds and conditions. In DC motors the winding current controls the motor torque and can be directly measured to achieve accurate control and the desired operation.
In an AC induction motor the torque is a function of induced current in the rotor which in turn is a function of slip, i.e., the difference in speed between the rotor and the rotating magnetic field produced by the stator. The speed of the rotating magnetic field is determined by the frequency of the winding energizing current. However, due to the slip, the rotor speed will differ therefrom by a variable amount related to the torque demands on the motor. Accurate control of rotor speed of an AC induction motor is difficult to achieve under variable torque conditions. Thus, even though AC induction motors are considerably less expensive than DC motors, they have generally not been used where accurate speed control is required.
One prior approach to servo speed control of induction motors is the "vector control" approach described, for example, in U.S. Pat. Nos. 3,593,083 and 3,824,437. Servo control of the induction motor is achieved by sensirg magnetic field conditions in the motor airgap or by deriving the field vector values from the stator voltage and current vectors. An inverter is then controlled in accordance with the filed vector values to supply an energizing signal to the motor having the desired phase, frequency and amplitude. Although this system functions well at running speed under load, this approach is characterized with poor control at low speeds and light loads. Under such conditions the magnetic fields in the motor are relatively weak or nonexistent and difficult to sense accurately. The calculated field vectors require integrations and therefore do not provide useful control informatin at zero speed. As a result, effective control based on these parameters cannot be achieved at low speeds. Furthermore, excessive power usage at low speeds results in undesirable heating.
Another approach is disclosed in patent application Ser. No. 297,809 filed Aug. 31, 1981 by James S. Whited now U.S. Pat. No. 4,559,485 wherein slip factors are empirically determined for a particular motor and these slip factors are utilized to generate a synthesized sine wave energizing signal having the slip and amplitude required to produce the torque necessary for achieving servo speed control. This approach eliminates the need for sensing the magnetic field vectors in the motor airgap and provides effective control under load at running speed. This system, however, tends to become unstable and oscillate at light loads. Also, the transient control response is less than desired to match DC motor performance.
An object of this invention is to provide a servo control system for controlling an induction motor which is capable of effectively controlling the motor over a range of conditions including zero speed and no load.
Another object is to provide such a system with improved transient control performance.
Still another object of the invention is to provide an AC induction motor servo control system capable of providing stiff control at stall, i.e., a control system capable of producing torque to resist movement from a stationary shaft position.